In a once-through steam generator, the heating of a number of evaporator tubes leads to complete evaporation of the flow medium in the evaporator tubes in one pass. The flow medium—usually water—, after it has been evaporated, is fed to superheater tubes connected downstream of the evaporator tubes and is superheated there. The position of the evaporation end point, i.e. the boundary region between unevaporated and evaporated flow medium, is in this case variable and dependent on operating mode. During full-load operation of a once-through steam generator of this time, the evaporation end point is, for example, in an end region of the evaporator tubes, so that the superheating of the evaporated flow medium begins as early as in the evaporator tubes. A once-through steam generator, unlike a natural or forced circulation steam generator, is not subject to any pressure restrictions, and consequently it can be designed for live steam pressures well above the critical pressure of water (PCri≈221 bar), where it is not possible to distinguish between the water and steam phases and therefore phase separation is also not possible.
Once-through steam generators of this type can be used in gas and steam turbine installations, in which the heat contained in the expanded working medium or heating gas from the gas turbine is utilized to generate steam for the steam turbine. Use may be envisaged in particular in combination with what is known as an industrial gas turbine with a rated
power of up to approximately 60 MW. With concepts of this type, in view of the boundary conditions which are predetermined by the nominal power, it is possible to provide for the preheating and evaporation of the water and the further superheating of the steam which is generated in a single once-through heating surface, the tubes of which are connected on the inlet side to entry manifolds for the supercooled feedwater and on the outlet side to exit manifolds for the superheated steam.
In low-load operation or when starting up a once-through steam generator of this type, the hot exhaust gas from the gas turbine is usually first of all passed to the uncooled tubes of the superheater section of the once-through steam generator, which for this reason usually have to be made from high-quality thermally stable materials. Alternatively, it is also possible for the evaporator section to be fed with a minimum flow of flow medium in order to ensure reliable cooling of the steam generator tubes. In particular at low loads of, for example, less than 40% of the design load, the once-through mass flow through the steam generator tubes corresponding to the associated steam power is usually no longer sufficient to cool these tubes, and consequently an additional throughput of flow medium is superimposed on this once-through passage of flow medium through the evaporator. In this case, separation of water out of the flow medium is usually required before the flow medium enters the superheater section of the once-through steam generator. For this purpose, the once-through heating surface in its entirety may be formed by an evaporator once-through heating surface, which is arranged in a heating gas passage and is formed from a number of evaporator tubes, and by a superheater heating surface, which is connected downstream of the evaporator once-through heating surface on the flow medium side and is formed from a number of superheater tubes, a water separation system being connected between the evaporator once-through heating surface and the superheater heating surface on the flow medium side.
In once-through steam generators of this type, the evaporator tubes which form the evaporator section usually open out into one or more exit manifolds, from which the flow medium is passed into a downstream water-steam separator, where the flow medium is separated into water and steam, the steam being transferred into a distributor system connected upstream of the superheater tubes, where the steam mass flow is divided between the individual superheater tubes connected in parallel on the flow medium side.
In a design of this type, the intervening connection of the water separation system means that in start-up and low-load operation the evaporation end point of the once-through steam generator is fixed rather than—as in the case of full-load operation—variable. Consequently, the operating flexibility of this type of design of once-through steam generator is considerably restricted in low-load operation. Furthermore, in a design of this type, the separation systems generally have to be designed, in particular with regard to the choice of materials, to ensure that the steam in the separator is significantly superheated in pure once-through operation. The required choice of materials likewise leads to considerable restrictions in operating flexibility. With regard to the dimensioning and construction of the components required, moreover, the abovementioned design means that the water discharge which occurs in the initial start-up phase when the once-through stream generator is being started up, has to be entirely dealt with by the separation system and discharged into the expander via the downstream separation cylinder and the outlet valves. The resulting relatively large dimension of separation cylinder and outlet valves leads to considerable production and assembly costs.